Because concentrated constant-type non-reciprocal circuit devices can be miniaturized, they have been used as terminals for mobile communications systems. An isolator is disposed between a power amplifier and an antenna in a transmission stage of a mobile communications system to prevent an unnecessary signal from flowing back to the power amplifier, thereby functioning to stabilize the impedance of the power amplifier on the side of a load. A circulator is used in a circuit for dividing a transmission signal and a receiving signal, etc.
FIG. 10 shows the general structure of an isolator as one example of conventional non-reciprocal circuit devices. This isolator comprises a ferrite plate 38 having a garnet-type structure, three sets of central conductors 31, 32, 33 disposed in the vicinity of the ferrite plate 38, and a magnet 20 disposed opposite thereto for magnetizing the ferrite plate 38 Each central conductor 31, 32, 33 is constituted by two substantially parallel straight lines, and three sets of the central conductors 31, 32, 33 are overlapped at an angle of substantially 120° such that they are crossing each other in an electrically insulating state.
The central conductors 31, 32, 33 are connected in parallel to dielectric substrate pieces (capacitors) 51, 52, 53 functioning as matching circuits. Further, the central conductors 31, 32 are connected to input/output terminals (not shown), and the central conductor 33 is connected to a terminating resistor 50.
Each central conductor 31, 32, 33 is usually integrally formed, for instance, by a thin metal plate 36 as shown in FIG. 11. The thin metal plate 36 comprises three sets of central conductors 31, 32, 33 radially and linearly extending from a ground electrode 34 at an angle of substantially 120°.
A ferrite plate 38 is disposed on the ground electrode 34 of the thin metal plate 36, and each central conductor 31, 32, 33 is folded on an upper surface of the ferrite plate 38 with an insulating sheet (not shown) therebetween, such that a tip end of each central conductor 31, 32, 33 projects outward from a periphery of the ferrite plate 38 to provide a central conductor assembly 30 shown in FIG. 12. The angles θx, θy, θz between adjacent pairs of central conductors 31, 32, 33 are usually 120°.
The central conductor assembly 30 is received in a center opening 100 of an insulating case 60, and capacitors 51, 52, 53 are received in the corresponding recesses of the insulating case 60. The insulating case 60 containing the central conductor assembly 30 and the capacitors 51, 52, 53 are contained in upper and lower magnetic metal cases 11, 12.
FIG. 13(a) shows the operation of a circulator, and FIG. 13(b) shows the operation of an isolator. The circulator is a non-reciprocal circuit device having three ports P1 to P3. A high-frequency signal flows from a port P1 to a port P2, from the port P2 to a port P3, and from the port P3 to the port P1, respectively, such that it circulates them. If the port P1 acts as an input port, the port P2 acts as an output port. In an ideal circulator, a signal introduced into the port P1 is not output from the port P3, while a signal introduced into the port P2 is output from the port P3.
The isolator has a structure in which a port P3 is connected to a terminating resistor Rt. Though a signal is transmitted from the port P1 to the port P2, a reflection signal from the port P2 to the port P1 and a signal introduced into the port P2 are transmitted by impedance mismatching to the port P3, in which they are consumed as heat by a terminating resistor Rt.
The ports P1, P2, P3 are called an input port, an output port, and an intermediate port, respectively, or an input port, a coupling port and a terminating port, respectively. The ports P1, P2, P3 will be called an input port, an output port, and a terminating port, respectively, below without intention of limitation.
The electric characteristics of the non-reciprocal circuit device are insertion loss and reverse-direction loss. The insertion loss is a loss generated when a signal passes from the input port P1 to the output port P2, and the reverse-direction loss is an insertion loss from the output port P2 to the input port P1 in the case of an isolator.
Particularly in a transmitting and receiving circuit used in cellular phones, etc., smaller power consumption results in a longer battery life. Therefore, it is preferable to use a device with low insertion loss. Accordingly, it is important that a non-reciprocal circuit device used in the transmitting and receiving circuit has as low an insertion loss as possible.
Referring to FIG. 14 showing the dependency of the circular polarization permeability μ of a garnet-type ferrite on an external magnetic field (DC magnetic field) Hdc, the microscopic operating principle of a non-reciprocal circuit device will be explained. Microwave signals introduced into the non-reciprocal circuit device comprise an electric field wave (E wave) and a magnetic field wave (H wave) perpendicular to each other, which are transmitted through the strip lines of the central conductor while vibrating. Because two waves perpendicular to each other have the same amplitude with phases deviated by 90°, a synthesized wave is circular vibration. Because a constant electric field changes its direction only, the synthesized wave is called circular polarization.
The permeability μ of a garnet-type ferrite differs depending on the rotation direction of a high-frequency magnetic field, which is represented by a complex permeability (μ′−jμ″). The imaginary part of the complex permeability represents loss. The permeability μ is represented by μ+′−jμ+″ in a positive rotation direction of a high-frequency magnetic field, and by μ−′−jμ−″ in a negative rotation direction of a high-frequency magnetic field.
The rotation angle φ of a high-frequency magnetic field is determined by the difference between (μ+′ and μ−′, namely μ+′−μ−′). When the external magnetic field is near a magnetic resonance Hr, a rotation angle φa at a magnetic field strength of Ha, for instance, is larger than a rotation angle φb when an external magnetic field is at a magnetic field strength Hb. This is because there is a large difference between μ+′ and μ−′ when the external magnetic field is near the magnetic resonance Hr, resulting in a large difference in inductance. Here, the rotation angle φ is an angle at which a plane of polarization rotates when a microwave signal proceeds along a magnetization direction.
When the external magnetic field is near the magnetic resonance Hr, a large rotation angle of a high-frequency magnetic field is obtained, though there is a large imaginary part μ+″ in a circular polarization permeability representing a loss component. As the external magnetic field becomes larger than the magnetic resonance Hr, the imaginary part μ+″ of the circular polarization permeability becomes smaller.
Paying attention to the imaginary part μ+″ of the circular polarization permeability, it has been found that what is needed to obtain a non-reciprocal circuit device with a small insertion loss is to apply a larger external magnetic field to set an operating point distant from the magnetic resonance Hr.
As described above, the operations of three ports P1, P2, P3 are conventionally made equal by setting the crossing angles of central conductors 31, 32, 33 to 120° in a non-reciprocal circuit device, thereby obtaining highly symmetric electric characteristics such as insertion loss, reverse-direction loss (isolation), reflection characteristics, etc. However, the miniaturization of a non-reciprocal circuit device and the reduction of insertion loss have been strongly demanded. To meet these demands, it has been proposed to increase an external magnetic field applied to a ferrite plate, and make an angle θz between the central conductor 32 connected to an input port P1 and the central conductor 31 connected to an output port P2 larger than 120° corresponding to the rotation angle of a high-frequency magnetic field, thereby causing the angles θx, θy, θz of the central conductors 31, 32, 33 to deviate from symmetry, such that a non-reciprocal circuit device is operated in an area in which a magnetic loss μ+″ is small (for instance, JP 9-102704 A, JP 10-112601 A, JP 10-163709 A). However, because a lower external magnetic field is preferable to improve a reverse-direction loss, the above conventional technology is disadvantageous in failing to reduce insertion loss.
In the case of an isolator, too, the deviation of the crossing angles of the central conductors from symmetry to make an angle θz larger inevitably results in angles θx, θy smaller than 120°, which are formed by the central conductors 32, 31 connected to the input port P1 and the output port P2 and the central conductor 33 to be terminated. Accordingly, a crossing angle of the central conductor 31 connected to the output port P2 and the central conductor 33 to be terminated does not correspond to the rotation angle of the high-frequency magnetic field. Further, a larger magnetic field than the optimum external magnetic field is applied to the central conductor 31 connected to the output port P2 and the central conductor 33 connected to a terminating port P3, resulting in larger impedance of the terminating port P3 than those of the input port P1 and the output port P2. As a result, matching fails to be achieved with a terminating resistor Rt, resulting in extreme deterioration of the reverse-direction loss.
Because power amplifiers less likely to cause intermodulation distortion are used in digital cellular phones, the non-reciprocal circuit devices may have relatively small reverse-direction loss. Nevertheless, the reverse-direction loss is required to be 6 dB or more, preferably 8 dB or more in a used frequency band.
Though the mismatching of impedance as described above can be dealt by matching the resistance of the terminating resistor Rt to the characteristic impedance of the terminating port P3, the reverse-direction loss is improved only in a narrower frequency band than the used frequency band, and it is less likely that the reverse-direction loss of 6 dB or more cannot be obtained in the used frequency band.
Turning to a means for applying an external magnetic field, a ferrite magnet has been used so far. Because a garnet-type ferrite has a saturation magnetization whose temperature coefficient is as large as −0.4%/° C. to −0.2%/° C., the use of a ferrite magnet having a large temperature characteristic of a residual magnetic flux density Br reduces the variation of high-frequency characteristics of a non-reciprocal circuit device at an ambient temperature. Best in magnetic properties among ferrite magnets commercially available at present is an SrLaO·(FeCo)2O3 ferrite magnet having a residual magnetic flux density Br of about 0.45 T and (BH)max of about 39 KJ/m3.
An external magnetic field applied to the ferrite plate is largely affected by the magnetic properties of the magnet 20 and its outer size. Non-reciprocal circuit devices widely used at present for terminals of cellular phones for mobile communications systems are 5 mm each with thickness of about 1.7 to 2.0 mm, containing, for instance, ferrite magnets of 4 mm each and 0.6 mm in thickness. However, it has been substantially difficult for a ferrite magnet to apply an external magnetic field corresponding to the angle of a central conductor more than 120° in a conventional non-reciprocal circuit device, because of the limitations of a ferrite magnet in magnetic properties, dimension and shape, etc.